Manufacture of insulating material



Patented Nov. 24, 1942 mmacruaa F Albert William Henry wedlock, Brookway, i ChigwelLEnglnnd 1 No Drawing No.

Application August-9,1940, 351,170. ,In'Glelt Britain Allfllst 1939 11 Claim. (01. 255156) I Thisinvention relates to the manufacture of micaceous insulating materials'ofthe kind which are formed by mixing low fusing point vitreous material with powdered mica and heating the mixture so that the mica is bonded to the vitreous material at a temperature below the point of total dehydration. Such materials and their manufacture are described, "for example; in U.;S. Patents Nos. 1,795,200, granted Mar. 3, 193 1, and 2,032,239, granted Feb, 25, 1936, and they are at present denoted bythe registered trade-mark Mycalex.- I

The object of the present invention is to "provide improvements in the process of manufacturing insulating materials of the kind above referred to, such improvements being designed to ensure a product whereinthe structure and physical characteristics are uniform throughout each unit manufactured. I, g

According to theinvention a method of manufacturing a vitreous insulating material is provided which consists in mixing ground mica with ground ceramic materials, in cold pressing the mixture, in drying the pressed sheets or blocks, in subjecting the dried sheets or blocks to a gradual rise in temperature in a furnace and. in cooling or moulding the heat treated material.

In carrying the invention into effect and in the v preferred manner, the ceramic ingredients employed in the manufacture of the improved vitreous material consist of cryolite, barium carbonate, potassium carbonate, soda ash and boric acid, the preferred proportions being by weight as follows:

Per cent Cryolite, 3NaF,A1Fa 20 Barium carbonate, BaCOa 10 Potassium carbonate, K200: 13 Soda ash", NarCOa 9 Boric acid, H3303 48 Alternatively the above ingredients may be varied by including a small proportion of felspar, not exceeding 5% by weight.

These materials are first ground to a fine powder, and after grinding and thoroughly mixing, these materials are passed through a drying furce in which a temperature of 500 C. is mainined until a white glass frit is formed.

The mica product is ground, care being taken grinding that the particle size is consistent and whilst the ground product should be capable of passing a mesh of 100 x 100 per square inch, the particles should not be smaller than this, as otherwise the finished product is apt to disintegrate under heat, with a corresponding loss. in di-electric properties. The frit is also ground tothe same .desreeasthe mics. and both are then mixed witha'quantity of heat treated Proportions of the'mix- Mycalex" the preferredture being as follows:

These materials in a .coldldry state are mixed for about five minutes, followed by a"furth'er period of mixing of'about 10 minutes, during which 8% by volume of water'ls sprayedupon balls of one or other-of the ingredients are found. to form during: the "mixing process. In'order to break downthese balls and'form a homogeneous mixture the mixed powders are pressed through a sieve of .5 mm. mesh and the mixture is also allowed to cool by this procedure in readiness for the pressing operation which follows.

The pressing of the mixture of mica, Mycalex and frit is carried out as a cold process, the

mould cavity being filled with the desired amount of powder according to the size and thickness of the billet or other article to be pressed.

In order that air pockets do not remain after I the'pressing operation, the operator kneads and stirs the powder in the mould cavity with ex-.

tended fingers until satisfied that the powder-is evenly distributed over the area of the said cav-V ity. The base of the mould cavity and the top of the charge of powder are preferably covered, before pressure is applied, with sheets of paper v in order to overcome the suction effect produced when the platens of the press are parted, thereby preventing the laminae of the completed billet being drawn apart. Pressure is applied to the powder at approximately 1 tons per square inch and produces solid billets which are placed in racks to dry over a period up to seven days,

this drying period being essential, as if placed in a furnace while still in a damp condition, blisters are liable to form on the surfaces of the billet as the result of the heat treatment which blisters crack and open to expose the laminae to oxidisation instead of forming a homogeneous mass.

The operation of cold pressing is carried out slowly so as to expel air gradually from the mass 56 of powder being pressed and it has been found 

